I have read the following from Swen Müller and Paulo Massarani, Transfer-function measurement with sweeps, Journal of The Audio Engineering Society 49 (2001), no. 6, 443–471.
“Another problem is ripple, which occurs at low frequencies. As mentioned previously, the multipliers produce sum and difference terms of the “time-delayed” excitation signal and the incoming response. At higher instantaneous frequencies, the sum is sufficiently high to be attenuated by the output low-pass filter. But at the low end of the sweep range, when the sum is close to or lower than the low pass cutoff frequency, “beating” will appear in the recovered magnitude response. To remedy this, the sweep can be made very long and the low-pass cutoff frequency reduced by the same factor. The better method, however, is to repeat the measurement with a “mirrored” setup, that is, exciting the DUT with a cosine instead of a sine. The real part of the complex result of this second measurement is added to the real part obtained by the previous measurement, while the imaginary part is subtracted. The effect of this operation is that the sum terms of the mixer output will be cancelled.”
Can the TEF perform this trick to get better LF resolution ?
Isn't it why we sometimes heard the reversed sweep go downward, then upward for a short time, at the end of the weep ? (with this kind of setup: reverse sweep, long sweep time, low frequencies)
Thanks a lot for the info